A Monocarboxylate Permease of Rhizobium leguminosarum Is the First Member of a New Subfamily of Transporters

Abstract

Amino acid transport by Rhizobium leguminosarum is dominated by two ABC transporters, the general amino acid permease (Aap) and the branched-chain amino acid permease (Bra). However, mutation of these transporters does not prevent this organism from utilizing alanine for growth. An R. leguminosarum permease (MctP) has been identified which is required for optimal growth on alanine as a sole carbon and nitrogen source. Characterization of MctP confirmed that it transports alanine ( K _m = 0.56 mM) and other monocarboxylates such as lactate and pyruvate ( K _m = 4.4 and 3.8 μM, respectively). Uptake inhibition studies indicate that propionate, butyrate, α-hydroxybutyrate, and acetate are also transported by MctP, with the apparent affinity for solutes demonstrating a preference for C ₃ -monocarboxylates. MctP has significant sequence similarity to members of the sodium/solute symporter family. However, sequence comparisons suggest that it is the first characterized permease of a new subfamily of transporters. While transport via MctP was inhibited by CCCP, it was not apparently affected by the concentration of sodium. In contrast, glutamate uptake in R. leguminosarum by the Escherichia coli GltS system did require sodium, which suggests that MctP may be proton coupled. Uncharacterized members of this new subfamily have been identified in a broad taxonomic range of species, including proteobacteria of the β-subdivision, gram-positive bacteria, and archaea. A two-component sensor-regulator (MctSR), encoded by genes adjacent to mctP , is required for activation of mctP expression.